linux-kernel - Re: [PATCH 3/5] LoongArch: Support relocation against _GLOBAL_OFFSET_TABLE

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Date:   Thu, 28 Jul 2022 09:02:57 +0800
From:   Jinyang He <hejinyang@...ngson.cn>
To:     Xi Ruoyao <xry111@...111.site>, loongarch@...ts.linux.dev
Cc:     linux-kernel@...r.kernel.org, WANG Xuerui <kernel@...0n.name>,
        Huacai Chen <chenhuacai@...nel.org>
Subject: Re: [PATCH 3/5] LoongArch: Support relocation against
 _GLOBAL_OFFSET_TABLE_

On 07/28/2022 12:28 AM, Xi Ruoyao wrote:
> With the stack-based relocations, the assembler emits three relocations
> to push the PC-relative offset of a GOT entry:
>
>      R_LARCH_SOP_PUSH_PCREL _GLOBAL_OFFSET_TABLE_
>      R_LARCH_SOP_PUSH_GPREL foo
>      R_LARCH_SOP_ADD
>
> "_GLOBAL_OFFSET_TABLE_" does not really exist in the symtab and the BFD
> linker handles it with special hack.  Implement a similar hack for
> kernel so we will be able to really use R_LARCH_SOP_PUSH_GPREL
> relocation.
>
> Signed-off-by: Xi Ruoyao <xry111@...111.site>
> ---
>   arch/loongarch/kernel/module-sections.c | 21 +++++++++++++++++++++
>   1 file changed, 21 insertions(+)
>
> diff --git a/arch/loongarch/kernel/module-sections.c b/arch/loongarch/kernel/module-sections.c
> index 509c0b86b1e9..73976addbf60 100644
> --- a/arch/loongarch/kernel/module-sections.c
> +++ b/arch/loongarch/kernel/module-sections.c
> @@ -89,6 +89,9 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
>   			      char *secstrings, struct module *mod)
>   {
>   	unsigned int i, num_plts = 0, num_gots = 0;
> +	Elf_Shdr *symtab = NULL;
> +	Elf_Sym *symbols;
> +	char *strings;
>   
>   	/*
>   	 * Find the empty .plt sections.
> @@ -100,6 +103,8 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
>   			mod->arch.plt_idx.shdr = sechdrs + i;
>   		else if (!strcmp(secstrings + sechdrs[i].sh_name, ".got"))
>   			mod->arch.got.shdr = sechdrs + i;
> +		else if (sechdrs[i].sh_type == SHT_SYMTAB)
> +			symtab = sechdrs + i;
>   	}
>   
>   	if (!mod->arch.plt.shdr) {
> @@ -115,6 +120,22 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
>   		return -ENOEXEC;
>   	}
>   
> +	if (!symtab) {
> +		pr_err("%s: module symbol table missing\n", mod->name);
> +		return -ENOEXEC;
> +	}
> +
> +	symbols = (void *) ehdr + symtab->sh_offset;
> +	strings = (void *) ehdr + sechdrs[symtab->sh_link].sh_offset;
> +
> +	for (i = 0; i < symtab->sh_size / sizeof(Elf_Sym); i++)
> +		if (symbols[i].st_shndx == SHN_UNDEF &&
> +		    strcmp(strings + symbols[i].st_name,
> +			   "_GLOBAL_OFFSET_TABLE_") == 0) {
> +			symbols[i].st_shndx = mod->arch.got.shdr - sechdrs;
> +			symbols[i].st_value = 0;
> +		}
> +
>   	/* Calculate the maxinum number of entries */
>   	for (i = 0; i < ehdr->e_shnum; i++) {
>   		int num_rela = sechdrs[i].sh_size / sizeof(Elf_Rela);

Hi, Ruoyao,


It is possible to create a symbol when linking, which means
maybe we can add _GLOBAL_OFFSET_TABLE_ in 'module.lds.h'.
What do you think about it?


Thanks,

Jinyang